Simple: DJs are live humans beings that create and match the energy and mood of the crowd- iPods do not know what is happening and cannot adapt. If you want your wedding to have life and excitement: hire a DJ. If you are just looking for background/ambient music, maybe an iPod is fine.
Determine what you want your wedding to be like for you and your guests. If you want people to be dancing and moving around, a DJ may be the way to go. This is especially true if the guest-list includes diversity of age, background, geography, gender, education, economy or culture. Diversity makes predictability of music challenging since the iPods play-list may not reflect what all the people want to enjoy. Remember; it is your day but everybody’s celebration.
Example: A birthday party for a six-year-old girl with only her friends from the neighborhood as guests probably will have a more predictable play-list than a wedding with guests from Korea, Ireland, Spain and Germany. The latter I did a few months ago and was amazed at the variety of music requested to meet the needs of this culturally diverse group of people celebrating a wedding. Truthfully, not only would a play-list on an iPod have been a failure, even carrying six racks of records back in the days when I still played vinyl would not have prepared me enough to make everybody satisfied the way they were. They had a great time and I needed to have the whole 125GB of my music library to be ready for the different styles and periods of music requested and honored. If you want your friends and family to be able to request and dance to the music they enjoy as well, please let them do so. We all know that having guests involved increases participation and energy at a wedding.
An iPod is perfect for a family dinner where everyone will sit at the table afterward talking and drinking coffee or maybe a pool party with just a select group of old friends as background music for catching up with each other. If you want people to be actively participating in your wedding; hire a good DJ and make it an event to remember and worth the huge effort and expense!